Conventionally, a shrink-fit tool unit is used in order to increase the sway precision of a rotary tool for machining. The shrink-fit tool unit includes a tool holder having a chuck in which a cylindrical terminal area of a rotary tool can be inserted, and holds the rotary tool by shrink-fit of the cylindrical terminal area with the chuck of the tool holder so as to transmit the rotary power to the rotary tool.
With this shrink-fit tool unit, it is possible to not only increase the gripping force of the rotary tool but also increase the sway precision in machining by minimizing the deflection of the rotary tool to be held, as compared with the case when the rotary tool is only pinched.
However, when machining load receiving from a work piece is large or, for example, when there is a constraint that the diameter of the cylindrical terminal area must be smaller than the root diameter of the cutting portion, such as for a tap, a sufficient gripping force cannot be obtained only with shrink-fit and consequently a slip is caused between the rotary and the tool holder.
To address this problem, in Japanese Patent Application Laid-Open Publication No. 2002-355727 for example, a shrink-fit tool unit is disclosed that includes a lock pin to restrain the rotation of the rotary tool held in the tool holder relative to the tool holder and thereby allows the rotary tool to be rigidly held without causing a slip between the rotary tool and the tool holder even when the machining load receiving from a work piece is large.